In the following description, it is assumed that a “legacy system” indicates a system which is conventionally defined and an “evolved system” indicates is a system evolved from the legacy system or a newly defined system.
It is assumed that “legacy support” indicates support to the legacy system in a transmission/reception relationship with the evolved system, and satisfies the following two conditions.
1) A legacy base station (BS) and a legacy mobile station (MS) can transmit/receive a signal to/from each other without having an influence on the evolved system, and the legacy BS and an evolved MS can transmit/receive a signal to/from each other.
2) A BS which can support the legacy system and the evolved system can transmit/receive a signal to/from the legacy MS and the evolved MS.
Hereinafter, for convenience of description, it is assumed that the IEEE 802.16e system is used as the legacy system and the IEEE 802.16m system is used as the evolved system.
First, the IEEE 802.16e system which is an example of the legacy system will be briefly described. At this time, it is assumed that the IEEE 802.16e system uses a 1024 FFT mode, that is, a 10-MHz bandwidth.
FIG. 1 shows a downlink subframe structure of the IEEE 802.16e system.
In the downlink subframe structure shown in FIG. 1, only a preamble related to the present invention will be described. The preamble corresponds to one OFDM symbol and is transmitted to an uppermost side of every frame. Such a preamble is used for time/frequency synchronization, cell search, channel estimation, or the like.
FIG. 2 shows a set of preamble subcarriers of a 0th segment in the IEEE 802.16e system.
As shown in FIG. 2, in the IEEE 802.16e system, both sides of a given bandwidth are used as protective bands, sequence is inserted in the remaining area at an interval of three subcarriers in consideration of three sectors corresponding to each segment, and 0 is inserted into a section into which the sequence is not inserted. A preamble of a first segment is inserted into frequency indexes 1, 4, 7, 10, . . . , 844, 847, and 849 and a preamble of a second segment is inserted into frequency indexes 2, 5, 8, 11, . . . , 845, and 848. If the sequence is inserted at the interval of three subcarriers in a frequency domain, the same effect as three-time repetition of the same sequence in predetermined units is obtained in a time domain, in view of the amplitude thereof.
That is, in the above-described IEEE 802.16e system, a preamble repetition factor is 3.
A portion of the sequence used in the preamble is shown by Table 1.
TABLE 1IndexIDcellSegmentSequence value (hexadecimal)000A6F294537B285E1844677D133E4D53CCB1F182DE00489E53E6B6E77065C7EE7D0ADBEAF110321CBBE7F462E6C2A07E8BBDA2C7F7946D5F69E35AC8ACF7D64AB4A33C467001F3B2220C75D30B2DF72CEC9117A0BD8EAF8E0502461FC07456AC906ADE03E9B5AB5E1D3F98C6E. . .. . .. . .. . .
As shown in Table 1, the sequence used in the preamble is set by a segment number and an IDcell parameter value. The defined sequence is converted into a binary signal in ascending order and is mapped to subcarriers by a BPSK modulation scheme. In other words, a hexadecimal sequence is converted into a binary sequence Wk and the binary sequence Wk is mapped from a most significant bit (MSB) to a least significant bit (LSB). At this time, 0 is mapped to +1 and 1 is mapped to −1. For example, since Wk is 110000010010 . . . 0th segment having an index of 0, the converted binary code values are −1 −1 +1 +1 +1 +1 +1 −1 +1 +1 −1 +1 . . . .
The sequence used in the preamble of the IEEE 802.16e system is composed of binary codes inserted in the frequency domain. This sequence is a sequence having a low peak-to-average ratio (PAPR), which is found by computer search when converting into the time domain while maintaining correlation characteristics to a certain extent, among the sequences composed of the binary codes. A preamble for the evolved system of the IEEE 802.16e system, that is, the IEEE 802.16m system, is not suggested in detail.
Accordingly, a method for utilizing the preamble for the IEEE 802.16e system which is the legacy system in the preamble for the IEEE 802.16m system without change may be considered. However, in this case, the following problems are generated.
First, in the IEEE 802.16e system, the sequence is inserted at an interval of three subcarriers in the frequency domain on the basis of a sector or a segment. For example, it is assumed that the preamble is transmitted via the subcarriers corresponding to 3k (k is an integer) in a sector 0, 3k+1 in a sector 1 and 3k+2 in a sector 2, and the remaining portion is transmitted via 0. A preamble reception signal having three-time repetition structure in a time domain is preferably subjected to time synchronization via auto-correlation. Accordingly, when such a repetition pattern is held even in the receiver, it is advantageous to acquire time synchronization of the receiver. The preamble for the IEEE 802.16e system is transmitted using a frequency location which varies according to sectors as described above. If preamble signals are repeatedly received from sectors 0, 1 and 2, the receiver cannot hold the repetition waveform in the received signal since the structure in which the sequence is inserted at the interval of three subcarriers is destroyed. Accordingly, in case of an MS located in the boundary between cells, time synchronization performance significantly deteriorates and complexity needs to be significantly increased.
If the evolved system such as the IEEE 802.16m supports a scalable bandwidth, a preamble based on 10 MHz used in the IEEE 802.16e is not suitable for the IEEE 802.16m. For example, the supportable bandwidth of the evolved system is 5 MHz, 10 MHz, 15 MHz and 20 MHz, the preamble for the IEEE 802.16e cannot be used in the system using a minimum transmission bandwidth of 5 MHz without change.
If both the legacy system and the evolved system are simultaneously supported, the two systems may be multiplexed by any one of time division multiplexing (TDM), code division multiplexing (CDM) and frequency division multiplexing (FDM). It is preferable that an evolved BS recognizes that an evolved MS is present in an evolved zone (or an evolved system support zone) without having an influence on a legacy BS and a legacy MS and knows a start location (that is, timing synchronization) of the evolved zone. However, it is difficult to support such a function if a legacy preamble is utilized as an evolved preamble without change or if only the legacy preamble is used. In addition, as described above, if the location or the start location of an evolved zone in a frame is not informed to the evolved MS, unnecessary delay may occur. For example, it is assumed that, if a BS for transmitting a legacy preamble is present and the evolved preamble is not detected during a predetermined time after an MS tries to search for the evolved preamble, a legacy preamble is searched for. In this case, since the MS searches for the evolved preamble at a time point when the legacy preamble is transmitted, a meaningless operation is performed over a significantly long period of time.
If the evolved preamble has the same repetition pattern as the legacy preamble, the MS may generate a false alarm in a preamble zone of a place where symbol synchronization is performed.